One of the easiest ways to tell your Kubernetes cluster where to put certain pods is to use a nodeSelector specification. A nodeSelector defines a key-value pair and are defined inside the specification of the pods and as a label on one or multiple nodes (or machine set or machine config). Only if selector matches the node label, the pod is allowed to be scheduled on that node.

While noteSelector provides a very easy way to control where a pod shall be scheduled, the affinity/anti-affinity feature, expands this configuration with more expressive rules like logical AND operators, constraints against labels on other pods or soft rules instead of hard requirements.

The feature comes with two types:

While Node Affinity is a property of pods that attracts them to a set of nodes, taints are the exact opposite. Nodes can be configured with one or more taints, which mark the node in a way to only accept pods that do tolerate the taints. The tolerations themselves are applied to pods, telling the scheduler to accept a taints and start the workload on a tainted node.

A common use case would be to mark certain nodes as infrastructure nodes, where only specific pods are allowed to be executed or to taint nodes with a special hardware (i.e. GPU).

Topology spread constraints is a new feature since Kubernetes 1.19 (OpenShift 4.6) and another way to control where pods shall be started. It allows to use failure-domains, like zones or regions or to define custom topology domains. It heavily relies on configured node labels, which are used to define topology domains. This feature is a more granular approach than affinity, allowing to achieve higher availability.

Descheduler is a new feature which is GA since OpenShift 4.7. It can be used to evict pods from nodes based on specific strategies. The evicted pod is then scheduled on another node (by the Scheduler) which is more suitable.

This feature can be used when:

Every wondered why Ansible Tower does not start downloading required collections when you synchronize a project? Here are the stumbling blocks we discovered so far:

As described at Compliance Operator the Compliance Operator can be used to scan the OpenShift cluster environment against security benchmark, like CIS. Fetching the actual results might be a bit tricky tough.

With OpenShift 4.8 plugins to the oc command are allowed. One of these plugin os oc compliance, which allows you to easily fetch scan results, re-run scans and so on. Let’s install and try it out.

OpenShift comes out of the box with a highly secure operating system, called Red Hat CoreOS. This OS is immutable, which means that no direct changes are done inside the OS, instead any configuration is managed by OpenShift itself using MachineConfig objects. Nevertheless, hardening certain settings must still be considered. Red Hat released a hardening guide (CIS Benchmark) which can be downloaded at https://www.cisecurity.org/.